The invention described herein relates generally to X-ray lasers, and more particularly to optical laser driven electron collisionally excited X-ray lasers.
During a period extending back from the present time for more than two decades, an intense search has been underway for ways of achieving, in the laboratory, laser emission at X-ray wavelengths. Coherent beams of X-rays produced by these lasers will have many beneficial scientific purposes, such as submicroscopic imaging, holography and spectroscopy. Although population inversions in plasmas of the type believed necessary to achieve X-ray lasing have been reported, no experiment providing conclusive or unrefuted evidence of the laser amplification of X-rays has yet been reported in the prior art.
One proposed generic high power optical laser-driven X-ray laser scheme that is currently being actively studied is the electron collisionally excited approach that is theoretically discussed by Elton in Applied Optics 14, 2243 (1975). This approach is also discussed by Zherikhin et al. in Sov. J. Quant. Electron. 6, 82 (1976). Zherikhin et al. theoretically speculate that electron collisions may establish a population inversion in ions with a ground electronic configuration 1s.sup.2 2s.sup.2 2p.sup.m as a result of 2p.sup.m-1 3s-2p.sup.m-1 3p transitions. The upper state 2p.sup.m-1 3p decays radiatively most effectively to the 2p.sup.m-1 3s state. While this lower state rapidly decays via a strong dipole transition, the radiative decay of the 2p.sup.m-1 3p state to the ground state 1s.sup.2 2s.sup.2 2p.sup.m is forbidden. Both of the states involved in the laser transition levels are thought to be primarily populated from the ground state as the result of electron impact. Additional pumping mechanisms, such as cascade from higher lying energy states, for example the 1s.sup.2 2s.sup.2 2p.sup.m-1 3d states, may be involved in the lasing process. Zherikhin et al. state that neon-like ions are most suitable for this scheme because they are relatively easy to produce and, in plasmas appropriate for high gain, they are the longest lived ions of those having the 2p.sup.m configuration. These features are caused by the large ionization potential of the filled 2p shell. Neon-like ions are atoms having an atomic number greater than ten that are stripped of all but ten of their usual complement of electrons. Zherikhin et al. calculate appreciable gains for plasmas composed of neon-like ions of elements in the atomic number range from 16 to 25 heated by two-stage laser pumping, wherein the electron component of a previously generated laser plasma filament is rapidly heated by an ultrashort pulse of high-power laser radiation traveling along the plasma filament. Because of theoretical difficulties, Zherikhin et al. state that it is not clear whether the method can be extended to high atomic number plasma systems.
Vinogradov et al., in Sov. J. Quantum Electron. 7, 32 (1977), consider the electron collisionally excited approach to high power optical laser-driven X-ray lasing under discussion and theorize that 3p-3s transitions can be inverted in optically thin, steady-state plasmas comprising neon-like ions carrying a charge between 7 and 15. Specific numerical results are given for the Ca XI ion. Calcium has the atomic number 20. An important finding of Vinogradov et al. is that two-stage laser pumping is not an absolute theoretical requirement of this electron collisionally-excited method.
As currently understood theoretically, the electron collisionally excited, single pass X-ray laser scheme involves using a driving conventional high power optical laser to produce a mid- to high-density plasma of neon-like ions. Strong monopole electron collisional excitation from the ground state of the neon-like ions fills 3p states. This inverts 3p-3s transitions because the lower energy 3s states radiatively decay very rapidly. Although the physics of the scheme is complex, it is nevertheless believed that strong 3p excitations may occur for neon-like ions produced from elements having an atomic number near 36 in systems driven by 0.53 micron wavelength laser light at an intensity of about 10.sup.13 to 10.sup.14 watts/cm.sup.2. The gain, usually stated in terms of reciprocal centimeters, of a transition produced by this scheme is believed to be a function of the parameters of the driving conventional high power optical laser pulse, the atomic number of the element comprising the plasma, the free electron density of the plasma, the electron temperature of the plasma, and, because of potential radiation trapping, the dimensions of the plasma. Two-stage laser pumping is not required.
An attempt was made to experimentally test the electron collisionally excited, single pass X-ray laser scheme at the NOVETTE laser facility of the Lawrence Livermore National Laboratory. The experimental arrangement is schematically shown in FIG. 1, prior art, to which reference is now made. Laser pulse 10, comprised of a 200 picosecond full width at half maximum amplitude, 0.53 micron wavelength, cylindrically focused light pulse having an average intensity of approximately 10.sup.14 watts/cm.sup.2, was directed onto a selenium panel 12, which was approximately 1,000 Angstroms thick. Selenium panel 12 was coated on a parylene substrate 14, which was approximately 0.5 microns thick. Parylene substrate 14 was supported within an aluminum trough 16. Laser pulse 10 caused a plasma, formed from blown-off selenium atoms, to come into existence adjacent to selenium panel 12. According to calculations performed on the Lawrence Livermore National Laboratory LASNEX computer code, and other computer codes, conditions within the plasma should have been such as to produce lasing emission at approximately 68 eV from neon-like selenium atoms by the electron collisionally excited mechanism. More particularly, according to the calculations, the plasma was expected to have, over an extended period of time, the electron density and gain, as functions of the distance from the surface of selenium panel 12, shown in FIG. 2, prior art, and in FIG. 3, prior art, respectively. During the lasing time, the plasma was expected to have an electron temperature of about 800 to 1500 eV. An X-ray detector which was carefully adjusted to measure radiation in the 58 to 78 eV energy range that was within an approximately 0.005 radian acceptance angle, monitored axial radiation emission, in a direction through the plasma and parallel to the surface of selenium panel 12, from the high gain portion of the plasma that was confined within approximately 30 microns of the surface of selenium panel 12. Laser amplification was not detected. Since the plasma was optically thin on the 2p-3s line coupling the ground state to the lower laser state, radiation trapping is an unlikely cause of this null result. Radiation trapping is the physical effect whereby the resonant line radiation resulting from the decay of the lower laser state is appreciably trapped within the laser medium and quenches the laser action. More specifically, radiation trapping can result in the interior portions of thick laser media having a very reduced intrinsic efficiency. The effect occurs when the very fast and fully allowed dipole transition radiation that empties the lower lasing state is reabsorbed by another ground state atom, thereby establishing an equilibrium which tends to elevate the population of the lower state of the lasing transition, destroy the population inversion, and terminate lasing.
Consequently, at the present time, while calculations such as that described herein have shown the mere theoretical feasibility of producing plasma media having gain in the X-ray region of the spectrum, no substantial X-ray laser amplification has yet been demonstrated. For such a demonstration to be clear and unequivocal, the product of the gain multiplied by the effective length of the X-ray laser amplifying medium should be at least as large as approximately three or four.
Thus, even though the theory underlying the generic electron collisionally excited single pass high power optical laser-driven X-ray laser mechanism is believed to be valid, it is not known in the prior art how to construct an operational X-ray laser of this type.